Without the ability to examine your son, review his MRI, and obtain a full history,
I can not comment on whether or not in his particular case he his arachnoid cyst is causing
bipolar disorder, but I will try to provide you with useful information.
Arachnoid cysts are a collection of cerebrospinal fluid (CSF, the fluid that covers the brain and spinal cord) between two layers of tissue called the arachnoid. They are thought to be present from birth in the large majority of patients.